Radio Frequency Identification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be predominantly electric or magnetic in the near field.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna section, a radio section, a power management section, and frequently a logical section, a memory, or both. In some RFID tags the power management section includes an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or battery-assisted tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device such as a battery, and are called passive tags. Regardless of the type, all tags typically store or buffer some energy temporarily in passive storage devices such as capacitors.
Tags are sometimes removed from tagged items, such as at point-of-sale or when the item is removed from its tagged packaging. Tags sometimes remain on tagged items, for future uses such as item returns to a store or in tagged identity cards. In some cases, especially when the tag remains on the item, the owner of the item may not want unauthorized readers to be able to read or track the item, such as for privacy reasons. Most conventional tags are always capable of being inventoried; those that inhibit regular inventory typically require a password-based challenge-response authentication with a reader before allowing themselves to be inventoried. The former tag types pose privacy risks to their owners; the latter tag types require complex password-based authentication that adds complexity to the reader and to the tag and makes it difficult to use the tags unless the interrogating reader has knowledge of both the authentication algorithm and the tag's secret password.